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This invention relates to optical switches or modulators and more particularly to an ultra-high speed optical modulator suitable for optical fiber transmission systems.
In order to extend the transmission distance at data rates of several Gb/s, a transmitter module with an external optical modulator is widely used due to its controllability and low chirp characteristics in order to minimize eye pattern degradation induced by the fiber dispersion. A semiconductor optical modulator is attractive because of its potentially small size and its low drive voltage. In a prior investigation reported in an article by Prof. F. Koyama et al., entitled xe2x80x9cFrequency Chirping in External Modulators,xe2x80x9d IEEE Journal of Lightwave Technology, Vol. LT-6, No. 1, January 1988, it was proposed that an optical directional coupler modulator might be able to provide both zero chirp and controllable chirp characteristics.
In an article by Dr. R. C. Alferness et al., entitled xe2x80x9cHigh-Speed Traveling-Wave Directional Coupler Switch/Modulator for xcex=1.32 xcexcm,xe2x80x9d IEEE Journal of Quantum Electronics, Vol. QE-19, No. 9, September 1983, it was reported that a simple directional coupler modulator using Ti:LiNbO3 material had operated at 7.2 GHz. As modulation frequency increases to the order of several GHz, a traveling wave electrode configuration is preferable because the cut-off frequency is not limited by a parasitic RC time constant.
The structure of a prior reverse delta beta type directional coupler modulator 2 is shown in FIG. 1. The device 2 includes a pair of waveguides 10, 11 having mutually parallel electrodes 12, 13, 14, 15 in sufficient proximity for delta-beta switched directional coupling. To operate such a device, two electrical modulation signals from sources 16, 17 with opposite signs are required. For L larger than the coupling length of the directional coupler and shorter than three times the coupling length, cross state and bar state are obtained for specific applied voltages Vc and Vb. In the cross state, when the bias voltage is Vc, the input light (optical radiation) of upper waveguide 18 is split into an upper and lower waveguides 10 and 11 at the end of what is the first directional coupler with 50% of input power distributed to each. Then by the reciprocity of reverse delta-beta type directional coupler modulator, the output light (optical radiation) comes out from only the lower waveguide 19 at the end of what is the second directional coupler. In the bar state, when the bias voltage is Vb, the input light of an upper waveguide 18 propagates only to the upper waveguide 20 at the end of the second directional coupler because of a larger phase mismatch. Accordingly both cross and bar state can be controlled completely by signal voltages with a wide fabrication tolerance of the structural parameters.
Dr. R. G. Waker et al. in an article entitled xe2x80x9cLow-voltage, 50 xcexa9, GaAs/AlGaAs Traveling-wave Modulator with bandwidth exceeding 25 GHz,xe2x80x9d Electronics Letters, 9 November 1989 vol. 25 No. 23 pp. 1549-1550, have proposed a traveling-wave Mach-Zehnder electro-optic modulator to provide periodic capacitive loading to a separate coplanar transmission line. Such a design is depicted by FIG. 2, which shows a prior art reverse delta beta type directional coupler modulator 4 using such a traveling-wave electrode configuration. A modulation signal is supplied by one signal generator 21 and terminates at a load 22. In order to apply opposite sign signals for a first directional coupler 23 and a second 24 directional coupler, the traveling-wave electrodes 25, 26 must be bent and crossed at the center 5 as shown in FIG. 2. It is also strongly desired to completely isolate the first and second directional coupler electrically to obtain efficient phase mismatches. However it is very difficult to realize this structure without degradations of RF wave and optical transmission characteristics. The crossing of traveling-wave electrode induces the reflection of the RF wave and weak grounding.
However, a simple directional coupler modulator is difficult to fabricate because the chip length needs to be precisely controlled to be an odd number multiple of the coupling length to yield a good extinction ratio. U.S. Pat. No. 4,012,113 disclosed that a reverse delta-beta type directional coupler modulator is suited for practical application due to a wide fabrication tolerance. This reverse delta-beta type directional coupler modulator requires two sets of control elements to achieve a phase mismatch of opposite sign. However, it is difficult to form a traveling wave type electrode configuration using this structure. Thus, what is needed is an optical modulator with both a traveling wave electrode configuration and a directional coupler with a wide fabrication tolerance.
According to the invention, a directional coupler type optical modulator with traveling-wave electrodes is provided which includes a waveguides forming a first directional coupler region, a wave coupling region, typically a waveguide crossover coupling region, a second directional coupler region, and a set of traveling-wave electrodes which are without crossover and are outside and adjacent the waveguides. The electrodes of each directional coupler are preferably connected to the traveling-wave electrodes via air-bridges. The waveguide structures are of the P-I-N type having a common N-type conducting layer which provides delta-beta operation of the directional coupler, and both cross and bar states are controlled by a single input signal. The proximate regions operate as a directional coupler controlled by the waveguide electrode bias voltages provided through the traveling wave electrode, such that they switch the optical signals propagating through the waveguides to each other. Two directional couplers are cascaded through a waveguide cross coupling region. The traveling wave electrodes propagate high-speed electronic signals from a signal input pad to the waveguide electrodes through the air-bridge structures. These two sets of directional couplers with waveguide cross coupling and traveling wave electrodes operate as a complete optical modulator.
According to the present invention, a reverse delta-beta type directional coupler modulator is formed by use of a single traveling-wave electrode configuration without crossover of the electrodes. By use of a crossing waveguide instead of a crossing electrode between two directional couplers, large electrical losses and reflection by the electrical crossing in RF range can be eliminated. A directional coupler, a Multi-Mode Interference (MMI) coupler or an X coupler using an optical crossing according to the invention produces low loss and negligibly small reflection characteristics. As a result, a high cut-off frequency and a very low drive voltage modulator can be achieved using the reverse delta beta type directional coupler modulator configuration.
In a specific embodiment of the present invention, the first pair of electrodes or second pair of electrodes on the optical waveguides is divided and disposed along the longitudinal axis, and each electrode is connected by an air-bridge individually to the outer traveling wave electrode. By configuring the electrodes on the waveguide as described, the electrical signal effectively propagates from the outer traveling wave electrode to each electrode on the optical waveguide when the period spacing of the electrode is smaller than the length of a period of the input electrical signal. As a result, this configuration achieves a small electrical attenuation even for a longer optical modulator.
Another specific embodiment of the present invention is one employing a ridge waveguide as the optical waveguide. According to the invention, the propagating light is well confined in an I-type low loss core region of the optical waveguide to reduce optical propagation loss so that coupling length of the pair of waveguides is easily controlled.
Another specific embodiment of the present invention is one employing a ridgeless waveguide as the optical waveguide. Optical coupling can be partially controlled by spacing between the waveguides sharing a common layer.
Another specific embodiment of the present invention is one employing a buried-hetero-structure (BH) waveguide as the optical waveguide. According to the invention, the propagating light is well confined in an I-type low loss core region of the optical waveguide to reduce optical propagation loss so that coupling length of the pair of waveguides is easily controlled.
Another specific embodiment of the present invention is one employing a directional coupler with cross state or a 2xc3x972 MMI coupler or a X waveguide as the crossing waveguide. According to the present invention, a low loss crossing region of the two waveguides in a short length can be fabricated.
Another specific embodiment of the present invention is one wherein divided electrodes are isolated from each other by removing the doped upper waveguide layer. According to the present invention, the electrodes are completely isolated and a leakage current between each electrode is suppressed effectively. In this case, the electrical loss of traveling wave electrode is further reduced.
Still another specific embodiment of the present invention is one employing on bias electrode which is deposited on the conducting layer on the semiconductor substrate. According to the present invention, an under layer of a pair of waveguides of the first directional coupler and the second directional coupler can be biased independently.
Still another specific embodiment of the present invention is one wherein the optical modulator is composed of Class III-V compound materials or Class IV compound materials. According to the present invention, efficient refractive change or phase change is achieved and a low loss optical modulator is achieved by a well-confined layered structure.
The scope of the present invention will become apparent from the following detailed description when read in conjunction with the attached drawings.